Refocus for filters?

[The Warthog]

I find that when I put a filter on the eyepiece, and pop it back into the focusing tube, I have to tweak the focus a little when I look through it again. I can't think of any logical, optical reason this should be so. Does anyone else get this? I have wondered if I'm moving the focuser when I change eps, but it is fairly rigid.

Not a big deal, just one of life's great mysteries.

[OXO]

When you place the Filtered eyepiece in your adjusting the distance of the Eyepiece to the point of focus on the telescope thus you will always need to do this. This is at least the only reason i can think of for having to do this..

James